1. Field of the Invention
The invention relates to the field of image processing. More specifically, the invention relates to a method for removing flicker from an image sequence.
2. Description of Related Art
Flicker is a term that refers to both global and localized temporal fluctuations in luminance amplitude, contrast, and chrominance amplitude (in the case of color sequences) in a motion picture image sequence. These fluctuations between one frame, a single image in the sequence, and another frame can be random, quick, and uncorrelated to one another. The image sequence can be in black and white or color, or even a combination of black and white separations combined to produce a color sequence.
Flicker can be caused by a variety of factors including: degradation of the recording media, e.g., film negative; variations in the density of the recording media caused by variations in development or exposure; variations in the light, both in luminance and chrominance, while recording the image sequence; and ac variations due to fluorescent lighting or ac variations present in the recording media. Also, flicker can result from disparate storage conditions and recording conditions for separations in the case where black and white separations are combined to make a color sequence. In addition, flicker can result from improper cleaning of the recording media, e.g., when cleaning the media with improper chemicals, which form globules on the surface of the media. The globules mottle the surface of the recording media and cause random variation in the density of the media, and, thus the quality of the image sequence.
Attempts have been made to correct for flicker by equalizing values of histograms of the intensity of the luminance, contrast, or chrominance, or by equalizing the mean frame value of the luminance, contrast, or chrominance, for consecutive frames. However, these methods do not take into account changes in scene content. Also, these methods are unable to compensate for motion within the image sequence, and they are unable to compensate for spatially localized flicker.
In addition, attempts have been made to correct for flicker by determining the mean and the variance of the luminance, contrast, or chrominance, for individual pixels within predetermined blocks of an image, and then correlating the blocks in different frames. The resulting correction that is applied to a block of an image is a linear multiplicative term that simply is calculated from the ratio of the blocks under examination. However, the large discrepancy in noise, variance, and film grain between successive frames in an image sequence make this approach an unviable one. Another disadvantage associated with this approach is that it is valid only for small motions of the camera and for objects moving relatively slowly within a scene. A further disadvantage associated with this approach is that it does not utilize histograms to classify image data.
Accordingly, there is a need for a flicker removal method that takes into account changes in scene content including motion within a sequence in addition to spatially localized and global flicker. The present invention satisfies this need.